Such systems have a .lambda.-probe which is exposed to the exhaust gas of the internal combustion engine and which emits an output signal which represents a measure of the air ratio .lambda.. In particular, a .lambda.-probe is used, the characteristic of which has an essentially jump-like behavior in the region of .lambda.=1 (Nernst-type .lambda.-probe). Furthermore, the open-loop/closed-loop control system has a basic memory, a desired-value memory and a closed-loop control device. In the basic memory, fuel-metering times (for example, injection times for the injection valves of the internal combustion engine) are stored in dependence on operating parameters of the internal combustion engine and in the desired-value memory, desired values of the air ratio .lambda. are stored in dependence on operating parameters of the internal combustion engine. The closed-loop control device corrects the fuel-metering time read out of the basic memory in dependence on an output signal of the .lambda.-probe measured and on a corresponding desired value read out of the desired-value memory.
Low-pollutant vehicles are usually operated with a three-way catalytic converter arranged in the exhaust gas of the internal combustion engine. In order to ensure the optimum conversion rate of the catalytic converter, it is necessary that an air ratio of .lambda.=1 is almost exactly maintained, that is the air ratio .lambda. may only fluctuate by a particular permissible amount around the value of .lambda.=1 (so-called catalytic converter window). In actual closed-loop control systems, control is frequently effected not exactly to .lambda.=1 but to .lambda..congruent.1 (for example .lambda.=0.998). In the text which follows, the term ".lambda.=1 control" will still be used for reasons of simplification, this term also being intended to encompass .lambda..congruent.1.
If the arrangement of a catalytic converter is omitted, a further possibility for reducing particular pollutant components of the exhaust gases of an internal combustion engine consists in operating the internal combustion engine in the lean range (.lambda.&gt;1). Thus, a large decrease of the nitrogen monoxides (NOx) contained in the exhaust gas is achieved, for example, with an air ratio of .lambda.=1.4. The carbon monoxide content (CO) of the exhaust gas is already very low at air ratios from .lambda.=1. However, there is an increase in the hydrocarbon content (HC) of the exhaust gas with large air ratios (from .lambda..congruent.1.1). However, the driving characteristic of the internal combustion engine stands in the way of increasing the air ratio .lambda. and the possible reduction in the pollutant components. To achieve an adequate driving characteristic of the internal combustion engine in any operating phase, it is necessary to enrich the air/fuel mixture in particular operating phases (for example idling, full load) by increasing the fuel quantity added so that values of the air ratio .lambda. occur which, under certain circumstances, are less than 1.
To be able to reliably cover such a wide control range (.lambda..congruent.0.9 to 1.4) by closed-loop control techniques, it is necessary in accordance with the solutions available in the prior art to use several controllers or to achieve a switch-over between individual control ranges by means of elaborate circuit measures. A closed-loop control device for the mixture composition of an internal combustion engine with switchable control ranges for .lambda.=1 range and lean range is known from U.S. Pat. No. 4,594,984, in which the .lambda.=1 control is effected by means of a two-position controller and lean control is effected either via an altered desired value of the two-position controller or with the aid of a constant controller.
The invention is based on the object of improving an open-loop/closed-loop control system for setting the air/fuel mixture, particularly for a control in the lean range.